1. Field of the Invention
The present invention relates to a method of and an apparatus for greatly inducing and fixing a photoinduced anisotropy (reorientation, birefringence and dichroism) of a medium by the irradiation of polarized light, as well as a photoinduced anisotropic medium prepared therewith and, particularly, it relates to a method of and an apparatus for preparing liquid-crystal-aligning films, wave plates, phase retarders, optical waveguides, non-linear optical elements and optical recording media by bit recording or holographic recording, as well as photoinduced anisotropic media thereof.
2. Description of the Related Art
The technique of orienting organic materials in solid state films have been studied vigorously in the application field of optoelectronics such as liquid-crystal-aligning films, as well as wave plates, phase retarders, diffraction gratings, optical waveguides, non-linear optical elements and optical recording. Among them, a method of controlling the orientation by the irradiation of polarized light has been noted particularly in recent years since this is a non-contact orientation method and capable of easily forming an optional orientation pattern in the film and has a possibility of application to optical devices utilizing active orientation change with light.
When a polarized light is applied to dichroic molecules in a state where molecular motion is restricted, only the molecules with the polarization axis and the transition dipole moment being aligned are excited selectively to cause optical anisotropy. This phenomenon is referred to as Weigert""s effect and reported in the 1920s. This is explained, for example, to azobenzene as a photoisomerizable molecule. Azobenzene shows trans-cis isomerization under the irradiation of light. It takes a molecular structure as shown in the chemical formula (a) in the trans-form, while a molecular structure as shown in the chemical formula (b) in the cis-form. 
Azobenzene shows anisotropy as an individual single molecule. But a film prepared by coating from a solution in a state they are bonded or dispersed in a polymer shows isotropy as a whole reflecting the isotropic conformation of the solution (refer to FIG. 1A). When a linearly polarized light at a wavelength to which azobenzene is sensitive is applied, only the molecules of azobenzene arranged in the direction identical with the polarizing direction absorb light due to the dichroism of azobenzene and are isomerized into a cis-form. Since the isomerized cis-form is thermally unstable, it is again isomerized into a transform by thermal back reaction. The trans-form in this case can be in any of orientation states identical with or perpendicular to the polarization direction (polarization axis), but the trans-form in the direction identical with the polarizing direction changes again by photoisomerization into the cis-form and, subsequently, is isomerized again into a trans-form by thermal back reaction. In the process of repeating the trans-cis-trans isomerization cycle defined by the polarization direction, change of orientation is caused in the direction of less absorption to the exciting polarized light (perpendicular direction to the polarization axis) (refer to FIG. 1B). In this case, change of the orientation of the polymer is induced by the isomerization and the change of orientation of azobenzene in addition to the change of the orientation of azobenzene to cause large anisotropy in the medium with the polarization direction as an axis (or in perpendicular thereto).
A method of manufacturing a liquid-crystal-aligning film or wave plate (optical phase retarder) utilizing such photoinduced anisotropy is proposed, for example, in Japanese Published Unexamined Patent Application No. Hei 11-231133. Further, as an optical recording method utilizing the recording characteristic of the photoinduced anisotropy, a holographic recording method (Japanese Published Unexamined Patent Application No. Hei 10-340478) or polarization multi-level recording (Japanese Published Unexamined Patent Application No. Hei 11-238251) are also proposed. Further, there are various application uses such as manufacture of optical waveguides or optical switches utilizing the change of refractive index by orienting non-linear chromophore such as azo dyes by irradiation of polarized light. In such cases, it is important to obtain a large birefringence by inducing and maintaining high orientation.
As a method of inducing higher orientation, in the method of light orientation using irradiation of polarized light, a method of using irradiation of polarized light together in the orientation by electric fields (Photoassisted Electrical Poling: PAEP) is proposed (Z. Sakkat, et al. xe2x80x9cPhotoassisted poling of azo dye doped polymeric films at room temperaturexe2x80x9d, Appl. Phys. B54, 486-489 (1992)). However, since an electrode is required in contact with the film surface for efficiently applying the electric field or it is difficult to form a fine optional orientation pattern, it is not practical in the application, for example, to liquid-crystal-aligning films or diffraction gratings.
As another method, Japanese Published Unexamined Patent Application No. Hei 11-160708 discloses a method of producing an oriented resin film of applying a linearly polarized light or a non-polarized light in an oblique direction and then further applying a heat treatment. This enables to transform the polymer into a liquid crystal or crystalline state and highly stabilize the orientation by the heat treatment. This is explained as a mechanism that the effective cooperative works among the polar and rigid azobenzene moieties induces a large orientation (K. Ichimura, et al. xe2x80x9cThermally stable photoaligned p-cyanoazobenzene moieties in polymer thin filmsxe2x80x9d, Macromol. Symp. 137, 129-136 (1999)).
However, since scattering attributable to the liquid crystal or crystalline state forms noises for optical elements in the application for thick films, for example, volume holographic memory, this is not suitable to polymers that have a liquid crystalline phase or a high crystalline property. Further, it is difficult to stabilize polymers that have no liquid crystalline phases or a low crystalline property by the heat treatment described above. FIG. 2 shows a result of preparing a film by using a polyester (refer to Chemical Formula 5), applying a linearly polarized light to a region of 1 mmxcfx86 thereby inducing and recording birefringence and applying a heat treatment to such a film to be described later. The effect of enhancing the orientation by the heat treatment is scarcely observed and it can be seen that the induced birefringence is remarkably attenuated at a temperature higher than the glass transition temperature (Tg=38xc2x0 C.) of this polymer. Further, since the effect of enhancing the orientation by this prior art essentially relies on the intermolecular force, it is considered that the effect is higher for uniform orientation in one direction but it is difficult to enhance fine orientation distribution at the submicron order by heat treatment.
When the change of the refractive index is applied to a wave plate, the thickness of the element can be reduced as the birefringence is larger and noises such as aberration depending on the element can be reduced. When a waveguide is manufactured, more modes can be propagated as the change of the refractive index is larger. Further, when manufacturing an optical switch having a waveguide layer in which non-linear chromophore such as an azo dye is oriented, second or third non-linearity can be increased by high orientation. Considering the application to the holographic memory, the recording multiplicity can be increased as the change of the refractive index is larger, and larger capacity memory can be attained. Also in the invention of a polarization multi-level recording (Japanese Published Unexamined Patent Application No. Hei 11-238251), if the change of the refractive index can be made larger, the thickness of the recording material can be reduced to decrease the effect of errors due to the change of the thickness of the medium.
However, in the light orientation method of the prior art described above (Japanese Published Unexamined Patent Application Nos. Hei 11-231133, Hei 10-340479, Hei 11-238251), the induced birefringence is about 0.05 which is not large enough for the application use such as orientation films, wave plates, optical memories or waveguides. For putting them into practical use, a method and an apparatus capable of maintaining the orientation stably and obtaining high birefringence are essential. Further, in the application for thick films, there is a subject in the scattering of the light orientation medium attributable to the liquid crystal or crystalline property. Furthermore, for the application to the holographic memory, a resolving power for forming and enhancing the orientation distribution at the submicron order is necessary.
xe2x80x9cOptical phase conjugation in polyesters with cyanoazobenzene units in the side chainxe2x80x9d by K. Nakagawa, et al, OPTICAL REVIEW 2, 460-462 (1995), shows that the polyester film having cyanoazobenzene in the side chain shown by xe2x80x9cChemical Formula 5xe2x80x9d to be described later is prospective as a phase conjugate mirror. The material generates a phase conjugated light by photoinduced anisotropy attributable to the photoisomerization of azo group and is capable of holographic recording. Further, the paper shows that the holographic grating is developed after the holographic recording. However, this mechanism was not clear.
This invention intends to provide a polymeric film having high optical anisotropy by a simple method, provide an apparatus therefor, as well as to provide a polymeric film having high optical anisotropy obtained by the method described above and an optical anisotropic medium having such a polymeric film.
This invention can be attained by the provision of the following aspects as the method, the apparatus and the optical anisotropic medium to be described later.
(1) A method of providing a polymeric film with optical anisotropy by applying a polarized light to a polymeric film containing a photoisomerizable group in the molecule or a polymeric film containing photoisomerizable molecules in which the photoisomerizable group or the photoisomerizable molecule has a T (thermal) type photochromic property capable of being isomerized by thermal back reaction after being isomerized by light, the method including two successive steps, that is, a step of applying a polarized light to the polymeric film and a step of shutting off the polarized light and leaving the same as it is, in which temperature of the polymeric film is controlled for enhancing the orientation of the photoisomerizable group or the photoisomerizable molecule induced by the thermal back reaction in at least one of the two steps.
(2) A method according to (1) above, wherein the temperature of the polymeric film is controlled in both of the two steps.
(3) A method according to (1) above, wherein the temperature is controlled such that the temperature of the polymeric film is within xc2x17xc2x0 C. of a glass transition temperature Tg of the polymer in at least one of the two steps.
(4) A method according to (1) above, wherein the temperature is controlled such that the temperature of the polymeric film is at the glass transition temperature Tg of the polymer in at least one of the two steps.
(5) A method according to (1) above, wherein, in the step of shutting off the polarized light and leaving the polymeric film as it is, the temperature of the polymeric film is controlled so as to be equal to or higher than the temperature of the polymeric film in the step of applying the polarized light.
(6) A method according to (1) above, wherein the time of applying the polarized light is set to xcfx84a or longer when the change of the photoinduced birefringence xcex94n relative to the polarization irradiation time t in the step of applying the polarized light is approximated by the following biexponential equation:
xe2x80x83xcex94n=Axc2x7{1xe2x88x92exp(xe2x88x92t/xcfx84a)}+Bxc2x7{1xe2x88x92exp(xe2x88x92t/xcfx84b)} (xcfx84axe2x89xa6xcfx84b)
where xcex94n represents a photoinduced birefringence, A and B each represents contribution of each of the components to the birefringence, xcfx84a and xcfx84b each represents a time constant for each of relaxation components and t represents the time for irradiation of the polarized light.
(7) A method according to (6) above, wherein the time of applying the polarized light is set to 6xcfx84a or longer when the change of the photoinduced birefringence xcex94n relative to the polarization light irradiation time t in the step of applying the polarized light is approximated by the biexponential equation.
(8) A method according to (1) above, wherein the temperature of the polymeric film is controlled by adjusting the irradiation intensity or irradiation power of the polarized light.
(9) A method according to (1) above, wherein the main chain of the polymer contains an aromatic hydrocarbon ring.
(10) A method according to (9) above, wherein the aromatic hydrocarbon ring is two or more benzene rings connected by way of a connection group.
(11) A method according to (1) above, wherein the photoisomerizable group or the photoisomerizable molecule contains an azo group.
(12) A method according to (1) above, wherein the polymer having the photoisomerizable group is a polymer in which an azobenzene derivative is introduced in the side chain.
(13) A method according to (9) above, wherein the polymer having the photoisomerizable group is a polymer in which an azobenzene derivative is introduced in the side chain, and the azobenzene derivative is connected to the aromatic ring of the main chain.
(14) A method according to (1) above, wherein the polymer having the photoisomerizable group is a polyester resin shown by the following structural formula: 
in which X represents a cyano group, a methyl group, a methoxy group or a nitro group, Y represents an ether bond or a ketone bond, 1 and m each represents an integer from 2 to 18 and n represents an integer from 5 to 500.
(15) A method according to (14), wherein the polymer having the photoisomerizable group is a polyester resin of the structural formula, in which 1 and m each represents an integer from 4 to 10 and n represents an integer from 10 to 100.
(16) An apparatus used for the method of providing a polymeric film with optical anisotropy by applying a polarized light to a polymeric film containing a photoisomerizable group in the molecule or a polymeric film containing photoisomerizable molecules in which the photoisomerizable group or the photoisomerizable molecule has a T (thermal) type photochromic property capable of being isomerized by thermal back reaction after being isomerized by light, the apparatus including a light source for applying a polarized light to the polymeric film and a temperature control unit that controls the temperature of the polymeric film.
(17) An apparatus according to (16) above, wherein the light source also serves as the temperature control unit.
(18) A polymeric film provided with optical anisotropy prepared by the method as described in (1).
(19) An optical anisotropic medium having a polymeric film provided with optical anisotropy prepared by the method as described in (1).
(20) A liquid-crystal-aligning film having a polymeric film provided with optical anisotropy prepared by the method as described in (1).